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‘Exocomets’ abound in alien solar systems

EXOPLANETS are used to the limelight – exocomets less so. Now, a fresh crop of comets found around alien stars suggests that these icy dirt balls stalk solar systems across the Milky Way.

Discs of debris swirling around young stars clump up to form planets. Asteroids and comets are the leftovers. Comets around alien suns are too small to see directly, so a team led by Barry Welsh at the University of California, Berkeley, looked for the chemical signatures of their tails as they are heated by their host stars. “Exoplanets are just so last year,” said Welsh at a meeting of the American Astronomical Society in Long Beach, California, on 7 January.

At the same meeting, Francois Fressin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, reported that 17 per cent of stars host a planet about the size of Earth, totalling 17 billion in the Milky Way alone. But these worlds orbit their stars too closely to be true Earth twins.